Oolitic Ironstones and Carbonate Mn Ores of the Marsyaty Deposit (Northern Urals, Russia): A Key Study of Mineralogy and Geochemistry

Abstract

This article describes the mineralogical and geochemical features of the Marsyaty Mn–Fe sedimentary deposit in the Northern Urals (Russia). Oolitic ironstones of the deposit are localized in the Cenomanian coastal sandstones. Manganese ores (carbonate and carbonate–oxide types) lies within the Lower Paleocene siliciclastic sediments and are separated from the oolitic ironstone (iron oxide and iron carbonate types) by a polymictic gravelite bed. Authigenic Fe³⁺ oxyhydroxides (goethite and ferrihydrite), chamosite/berthierine and late siderite predominate in the ironstones; kaolinite, apatite, perhamite, calcite and dolomite are minor constituents. Rhodochrosite and rancieite are the major minerals of the manganese ore; Mn-dominated phyllosilicates (parsettensite? and caryopilite?) are rare. Both ore types contain authigenic glauconite, montmorillonite, sulfides (sphalerite/wurtzite, galena, and pyrite), gibbsite/boehmite and REE phosphates. Both ore types contain detrital quartz, ilmenite, zircon, monazite, epidote, titanite, muscovite and feldspars. The δ¹³C_carb value (VPDB) varies from −18.5 to −23.3 in both ironstone types and from −10.0 to −41.0 ‰ in the manganese ore. The negative C isotopic composition and numerous organic remains indicate the involvement of microbial processes in the formation of both types of carbonate ores. The Fe and Mn ores belong to one transgression–regression sedimentation cycle and formed consecutively during the evolution of the West Siberian basin. A unique feature of the Marsyaty deposit includes two ore formation stages within a limited area and over a relatively short geological period: the accumulation of (i) oolitic ironstones enriched in Mn first and then (ii) manganese ores only.

1. Introduction

A combination of iron and manganese ores is typical of stratiform deposits associated with volcanic rocks and is much less common in deposits confined to sedimentary strata [1,2,3,4,5,6]. Oolitic ironstone bodies are rarely accompanied by individual manganese ores. Even if the oolitic ironstones contain higher Mn concentration, Mn usually enters the rock-forming Fe³⁺ oxyhydroxides (hereinafter, Fe means Fe³⁺), most often goethite, or forms Mn minerals (most often rhodochrosite), which are dispersed in oolitic Fe ores or form small lenses and beds inside Fe ores [7,8,9,10]. Thus, the Marsyaty Mn–Fe deposit in the Northern Urals (Russia) deserves attention because a carbonate manganese ore bed directly overlies the oolitic iron ore.

The Marsyaty deposit belongs to a group of Cretaceous–Paleogene sedimentary manganese deposits that occur within a narrow longitudinal band stretching for about 300 km along the eastern slope of the Northern Urals (Figure 1a). Fifteen Mn deposits, several occurrences, and eight potential areas within this belt form the Northern Urals manganese basin [1,11,12,13]. Although the Marsyaty deposit is mostly known as a Mn deposit, it contains bedded accumulations of oolitic iron ores, which formed on the periphery of the West Siberian sedimentary basin near its boundary with surrounding folded structures (Figure 1b). This area hosts numerous oolitic ironstone deposits, including the giant Bakchar and Ayat deposits, with similar ages and host strata lithologies (Figure 1b) [14,15]. Thus, manganese and iron deposits are ubiquitous throughout the regional sedimentary strata. However, areas of iron and manganese concentration in these deposits are typically separated in space and time and rarely occur together in the same deposits. The Ekaterinika ore manifestation, which is currently inaccessible, and the Marsyaty deposit are examples of such deposits that contain both iron and manganese ores (Figure 1a) [11].

The Marsyaty deposit was discovered in 1894 by the great Russian crystallographer Evgraf Fedorov. The manganese ores were exploited until the middle of the 20th century. The estimated reserves of the associated iron ores have no commercial value. Previous data based on exploration and exploitation work carried out during the middle of the 20th century are summarized in [11,12,17]. Our previous [18,19] and new mineralogical, geochemical and isotopic data on oolitic iron and carbonate manganese ores provided grounds for revising the existing view of iron and manganese accumulation in sedimentary basins.

The aim of this article is to provide a model for iron and manganese sedimentation and further diagenesis during the evolution of a pericratonic basin based on new mineralogical and geochemical data.

2. Geological Background

The Marsyaty deposit is located in the Sverdlovsk region (Northern Urals; 60.06778° N, 60.45167° E) (Figure 2a) and is confined to the Mesozoic–Cenozoic sedimentary cover of the West Siberian Platform, which overlies the Paleozoic volcanosedimentary sequences of the Urals folded system (Figure 2b). The host sedimentary rocks of the deposit formed in coastal continental and shallow marine environments (Figure 2c). The Paleozoic rocks are exposed ~1.5 km to the west.

The host rocks are composed of polymictic and glauconite–quartz gravelite, sandstone, sand, siltstone, and montmorillonite and beidellite claystone with glauconite. Phosphorite nodules are also found in the sedimentary section [11,12,17]. Two ore-bearing horizons that overlie each other and are separated by a bed of clastic rocks (gravelites) are established at the deposit (Figure 2d). The lower horizon, containing iron ores, is composed of Cenomanian coastal deposits (Figure 2c). The upper horizon is composed of Lower Paleocene siliciclastic sediments and contains carbonate manganese ores. Both ore horizons are heterogeneous and exhibit facies replacement by host sandstones, gravelites and sandy–clayey rocks. Both ore horizons strike longitudinally and dip to the east at angles of 5–15°, concordant with the general dip of the sedimentary strata. The ore-bearing unit is traced for 8.7 km along the strike and to a depth of 50 m. The thicknesses of the lower and upper ore-bearing horizons are 1.0–12.0 and 0.1–3.8 m, respectively. Currently, the deposit represents a chain of old open pits extending from south to north near the settlement of Marsyaty (Figure 2a).

3. Materials and Methods

The material for this study was collected from outcrops and ore stockpiles. Samples from outcrops were taken from various rock varieties across the bedding (Figure 3). A total of 12 iron ore and five manganese ore samples were studied in detail.

Minerals were first identified using optical microscopy and a TM 3000 scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan) equipped with an Oxford Instruments energy-dispersive spectrometer (EDS) (Oxford Instruments Plc, Abingdon, UK) at the Resource Center (RC) “Microscopy and Microanalysis” (Scientific Park (SP), St. Petersburg State University (SPbSU), St. Petersburg, Russia). The chemical composition of minerals was analyzed using a S-3400N SEM (Hitachi Ltd., Tokyo, Japan) equipped with an AzTec Energy X-Max 20 EDS (Oxford Instruments Plc, Abingdon, UK) at an accelerating voltage of 20 kV, a beam current of 10 nA, a beam diameter of 1–5 μm, and an accumulation time of 60 s in RC Geomodel (SP, SPbSU, St. Petersburg, Russia). The reference materials included native and artificial substances.

X-ray diffraction (XRD) analysis was carried out using a Shimadzu XRD6000 diffractometer (Shimadzu Corporation, Kyoto, Japan) (Cu-Kα radiation, a graphite monochromator, an angular range of 4–70° 2θ, and a canning rate of 2°/min) at the South Urals Federal Research Center of Mineralogy and Geoecology UB RAS (SU FRC MG UB RAS, Miass, Russia), and a Mini Flex II diffractometer (Rigaku Corporation, Tokyo, Japan) (CoKα-radiation, an angular range 5–60° 2θ, and a scanning rate 1°/min) at the RC X-Ray Analytical Methods (SP SPbSU, St. Petersburg, Russia).

The Raman spectra were acquired from polished sample surfaces in backscattering geometry over a range of 200–2000 cm^–1 using a Horiba Labram HR800 high-resolution spectrometer (Horiba, Kyoto, Japan) equipped with an Olympus BX41 microscope (Olympus, Tokyo, Japan) and an Ar⁺ laser with a working radiation frequency of 532 nm, a power of 50 mW, a grating of 1800 d/mm, with a range of 200–4000 nm, at the RC Geomodel (SP SPbSU, St. Petersburg, Russia).

The chemical composition of the ores was determined at the SU FRC MG UB RAS using the following methods: gravimetry (SiO₂), titrimetry (Al₂O₃, FeO, Fe₂O₃, CaO CO₂), photometry (TiO₂ and P₂O₅) atomic-emission spectrometry (Na₂O and K₂O), and ICP-MS (an Agilent 7700x mass spectrometer, Agilent, Santa Clara, CA, USA; MassHunter software package C.01.04., standards SGD-2a) (trace elements).

The carbon isotopic composition was analyzed on a Delta+ Advantage mass spectrometer (Thermo Finnigan LLC, San Jose, CA, US) equipped with a ConFlo III interface and an EA Flash1112 element analyzer using the Isodat 2.0 software and the reference material NBS19 (Limestone (Carbon and Oxygen Isotopes in Carbonate)), (reference value of NBS19: δ¹³C = +1.95 ‰, VPDB). was used for calculations. The measurement error was 0.08 ‰.

4. Results

The following lithological varieties are distinguished in the iron ore bed accessible for study (from top to bottom) (Figure 3):

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Dense, unsorted, medium- to fine-grained mostly quartz sandstone with limonite (a mixture of goethite and amorphous Fe oxihydroxides) cement and rare glauconite grains (15 cm thick),

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Loose quartz–smectite–limonite ocher (20 cm thick);

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Dense, mostly quartz sandstone with apatite–goethite cement, goethite and apatite–goethite ooids, and rare bean-like bauxite grains (20 cm thick);

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Loose goethite oolitic ironstone (1 m thick);

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Dense platy goethite oolitic ironstone (1.2 m thick);

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Dense, mostly siderite oolitic ironstone with calcite veinlets along fractures (visible thickness: 0.5 m).

The iron ores are overlain by a 0.5 m thick bed of coarse-grained, poorly sorted rock with ocher and clay cement, which is crowned by a bed of highly weathered carbonate–oxide manganese ores with a visible thickness of 0.5 m.

4.1. Petrography and Mineralogy

4.1.1. Iron Ores

The iron ores of the Marsyaty deposit are typical oolitic ironstones (Figure 4). They consist of ooids, litoclastic detritus and ocher cement. The ironstones form packages of alternating beds of more and less loose structure (Figure 4a,b). The beds differ in thickness and in the content of detrital components (Figure 4c,d), while exhibiting similar ooid sizes.

The amount of medium- and coarse-grained material varies from 10 to 70 vol. %. Large (5–20 mm across) fragments mostly include rounded quartz grains. Feldspar grains, fragments of granite and other rocks, mineralized plant remains and mollusk shells are less common. Glauconite globules, ilmenite, rutile (anatase?), zircon, epidote, monazite (rhabdophane?) and xenotime grains, together with rounded aggregates of Al hydroxides (bauxite), compose the finer (<1 mm) fraction. Ferruginous ooids in a fine mass of authigenic minerals occur between the detrital fragments. The diameter of the ooids ranges from 0.2 to 0.5 mm on average, rarely reaching 1 mm (Figure 5e,f).

Based on the mineral composition, the iron ores are divided into the main loose brown iron oxide (goethite–quartz–chamosite) and the subordinate denser (massive) greenish (on fresh sample surfaces) iron carbonate (kaolinite–quartz–siderite) types (

Table 1. Minerals of the iron and manganese ores of the Marsyaty deposit, modified after [18,19].

Mineral	Methods	Fe_ox	Fe_cb	Mn_cb	Mn_cb-ox
Sphalerite/wurtzite ZnS	2		+	+	+
Galena PbS	1, 2		+	+
Pyrite FeS2	1, 2		+	+
Chalcopyrite CuFeS2	1, 2		+
Quartz/Opal SiO2	1, 2, 3	■	■	●	●
Rutile/Anatase TiO2	2	+	+	+	+
Ilmenite (Fe,Mn)TiO3	2	+	+	+	+
Goethite FeO(OH)	1, 2, 3, 4	■	●		■
Ferrihydrite Fe5O7(OH)	3	+
Rancieite CaMn5O10 · 3H2O	1, 2, 3, 4				■
Gibbsite/boehmite AlO(OH)	1, 2, 3	+		+
Zircon Zr(SiO4)	1, 2	+		+	+
Titanite CaTi(SiO4)O	2				+
Epidote Ca2FeAl2(SiO4)(Si2O7)O(OH)	1, 2	+	+	+	+
Kaolinite Al2(Si2O5)(OH)4	2	+	●
Caryopilite ? Mn5(Si4O10)(OH)6	2			+
Chamosite/Berthierine Fe5Al(AlSi3O10)(OH)8	1, 2, 3	■			+
Glauconite K0.8(Fe,Mg,Al)2[(Si,Al)4O10](OH)2	1, 2, 3	●	+	●	+
Muscovite–phengite K(Al,Mg,Fe,Mn)2[(Si,Al)4O10)](OH)2	1, 2, 3	+		+
Parsettensite ? KMn7(AlSi9O24](OH)6 · nH2O	2			●
Montmorillonite K0.5(Fe,Al,Mg)2(Si4O10)(OH)2 · nH2O	2, 3	+	+	●	●
Albite Na(AlSi3O8)	2, 3			+	+
K-feldspar K(AlSi3O8)	1, 2, 3	+	+		+
Calcite CaCO3	1, 2, 3		+	+
Rhodochrosite MnCO3	1, 2, 3	+		■	■
Siderite (Fe,Mn)CO3	1, 2, 3	+	■	+
Dolomite CaMg(CO3)2	1, 2	+
Apatite Ca5(PO4)3(OH,F)	1, 2, 3	+	+	+	+
Monazite/rabdophane Ce(PO4)	2	+	+	+	+
Xenotime Y(PO4)	2	+
Perhamite (Ca,Sr)3Al7.7(Si3P4O23.5)(OH)14.1 · 8H2O	1, 2, 4		+

Ore types: Fe_ox, iron oxide; Fe_cb, iron carbonate; Mn_cb, manganese carbonate; Mn_cb-ox, manganese carbonate–oxide. Analytical methods of mineral identification: 1, optical microscopy; 2, electron microscopy and microanalysis; 3, XRD analysis; 4, Raman spectroscopy. Minerals: ■ major (>5 vol%); ● minor (1–5 vol%); + accessory (<1 vol%). ? minerals required refinement are questioned.

), which together form one bed. The iron carbonate ores are characterized by fractures perpendicular to the bedding. They also host thin calcite veinlets along the fractures.

The iron oxide ores consist mainly of authigenic goethite and subordinate quartz and chamosite. Almost half of the total amount of goethite is hosted in fine, concentrically zoned ooids (Figure 5a). Massive goethite, fragments of earlier zoned ooids, clasts of quartz, ilmenite and other minerals, and locally plant remains can be found in the ooid core. The cortex (outer concentric zones) most commonly consists of rhythmically alternating layers composed either of goethite with varying contents of trace elements (Al, Si, P, Ca and Mn) (Figure 5b) or of alternating goethite and chamosite. A cortex composed of chamosite with thin goethite zones is less common (Figure 5c). The ooids could contain more than one (two or more) nuclei with different composition. The ooids exhibit traces of mechanical deformation and rotation, with repeated deposition of iron matter. In some ooids, apatite inclusions highlight the zonal structure of cortex. The cement mostly consists of goethite and/or chamosite and, less commonly, apatite. The goethite grains between the ooids are often zoned (Figure 5d), with dense, homogeneous cores and porous thin rims containing microscopic quartz, chamosite, and apatite grains. Remains of microorganisms (algae?) are locally overgrown and replaced by goethite (Figure 5e).

Wide reflections and a high background in the XRD patterns of the ooid cortex, together with the diffuse character of the Raman bands, indicate a low degree of crystallinity of the Fe oxyhydroxides. The Fe oxyhydroxides in the cement have the most perfect structure, corresponding to crystalline goethite.

Goethite contains varying amounts of trace Si, Al, Mn, Ca, Mg and P. The highest contents of these elements are determined in goethite from the ooids (18–25 atomic %). In the iron oxides of the cement, the content of trace elements is up to 9 and 13 atomic % in the grain cores and the rims of the same grains, respectively. These iron oxides are characterized by the highest Mn content, which is consistent with the findings of rhodochrosite relics in the cement.

Glauconite, muscovite–phengite, montmorillonite, kaolinite, Al hydroxides (gibbsite and/or boehmite?), siderite, calcite, dolomite and rhodochrosite are minor minerals of the oxide iron ores (Table 1).

The iron carbonate ores are mostly composed of kaolinite, quartz and siderite, were siderite accounting 20 and more vol. %.

Siderite forms aggregates of two morphological types. Type 1 pseudomorphically replaces goethite and chamosite–goethite ooids. In the iron oxide ores, some siderite occurs as anhedral aggregates within goethite ooids (Figure 5f). Rarely, platy rhodochrosite crystals are intergrown with apatite in the cement. The iron carbonate ores contain rounded “shadows” of goethite ooids that are completely replaced by siderite. These siderite ooids no longer exhibit zoning. The former concentric structure can locally be recognized from relict goethite microinclusions or perchamite inclusions (Figure 5g). Siderite forms either lumpy aggregates or radial spherulites that are irregularly oriented within the former ooids. The spherulites can be located in both the ooids and the cement. The acicular siderite crystals composing the spherulites often cross the growth zones of the primary goethite ooids. Type 2 consists of interstitial small (~100 µm in diameter) lumpy spherulitic siderite aggregates occurring between the siderite ooids (Figure 5h). Rare interstitial acicular rhodochrosite is associated with apatite (Figure 5i).

The chemical composition of siderite of both morphological types is similar. The FeCO₃ content is 71–95 mol%. Siderite typically contains Ca, Mg, and Mn. Siderite with a high (up to 36 mol%) MnCO₃ content is less common. Platy rhodochrosite crystals contain 79 mol% MnCO₃.

4.1.2. Manganese Ores

Two types of manganese ores are identified at the Marsyaty deposit: (1) carbonate (rhodochrosite) and (2) carbonate–oxide (rhodochrosite–goethite–rancieite) (Figure 6). According to previous geological exploration results, the carbonate ores were dominant, but now they occur only as isolated blocks in former ore stockpiles. At the surface, the carbonate ores are extensively replaced by supergene Fe and Mn oxides and hydroxides along fractures and pores, but primary rhodochrosite aggregates remain in large ore samples. The primary carbonate–oxide ores are less typical in the deposit. They form small fragments at the base of the manganese ore bodies, one of which is now exposed in the wall of an old quarry above the iron ores (Figure 6a).

Carbonate (rhodochrosite) ores are light beige to light gray, fine-grained rocks with a typical stromatolite-like structure (Figure 6b–d). In cross-section, “the stromatolites” have a shell-like, wavy or truncated concentric-zoned structure formed by the rhythmic alternation of thin (1–2 mm thick), convex, curved laminae separated by small pores, which are partly or completely filled with supergene minerals. In cross-sections parallel to the elongation of the “stromatolite”, the carbonate beds are slightly curved, and the rock exhibits a uniform banded pattern.

The ore texture is lumpy and spherulitic (Figure 7a). The ore cement contains rhodochrosite spherulites and their intergrowths, up to 500 µm in diameter, with a clear concentric-zoned structure (Figure 7b) due to varying content of Mn, Fe, Ca and Mg, as well as the presence of fine Fe oxyhydroxide inclusions (goethite?) in some zones (Figure 7d). The intergrowths of spherulites are characterized by synchronous changes in the composition of the concentric growth zones of adjacent spherulites and by a outermost rim surrounding the spherulite aggregate. The average MnCO₃ content of spherulitic rhodochrosite is 65–70 mol%. A negative correlation is observed between Mn and Fe contents (r_Mn–Fe = –0.82, n = 41), whereas a positive correlation is established for Mg and Ca (r_Ca–Mg = 0.77, n = 41) (at the probability of 95% in both cases) (Figure 7e). Rhodochrosite in the main lumpy–spherulitic mass has a more uniform chemical composition. It contains, on average, 85 mol% MnCO₃. The cores of spherulites, up to 100 µm in diameter, locally have elevated Ca and Mg concentrations (up to 15 mol% CaMg(CO₃)₂).

The carbonate cement between the large spherulites has a globular, lumpy and locally microspherulitic structure. Numerous elongated or irregular pores, 5–50 µm in size, occur along the boundaries of the rhodochrosite globules and microspherulites.

Manganese carbonate ores contain numerous micropores, which are encrusted by carbonate crystals with high Fe and Mn concentrations: 34 mol% MnCO₃ and up to 56 mol% FeCO₃. A mineral with Fe > Mn should be referred to as Mn-bearing siderite. Compared to the rhodochrosite in the spherulites and the cement, the Mn-bearing siderite is a later mineral (Figure 7c).

Isometric globules of microflaky glauconite are often found within the pores, as well as lamellar grains or, more commonly, tangled–flaky clusters of phyllosilicates (muscovite–phengite, montmorillonite, parsettensite?, and caryopilite?) (Figure 7f). The phyllosilicate grains are also present in the cement, where they overgrow the lumpy rhodochrosite aggregates. The total amount of phyllosilicates does not exceed 1–5 vol%. The amount of quartz is similar. Sphalerite (wurtzite?), galena, pyrite, rutile (anatase?), ilmenite, Al oxide (gibbsite/or boehmite?), zircon, epidote, albite, calcite, apatite, and monazite are accessory minerals of the rhodochrosite ores.

Carbonate–oxide (rhodochrosite–goethite–rancieite) ores are black to brownish coarse-clastic (Figure 6e,f). Rounded and irregular clasts of quartz, metavolcanic rocks, schists and granites, 0.5–3.0 cm in diameter, compose 50–70 vol% of the ore. The clasts are enclosed in a micro- and fine-grained cement consisting of rhodochrosite and Mn and Fe oxides (Figure 7g). Small (50–200 µm across), partly rounded, isometric, elongated or irregular fragments of rhodochrosite spherulites are abundant in the cement. The rhodochrosite aggregates are overgrown by a rim (50–100 µm thick) of columnar rancieite (Figure 7h), which is typically oriented perpendicular to the rhodochrosite spherulites. The rhodochrosite–rancieite aggregates exhibit a cockade or microconcretion structure.

Some carbonate–oxide ore samples have a rhythmically bedded texture, whereas their textures and composition are similar to those of the coarse-clastic carbonate–oxide ores. The bedded carbonate–oxide ores, however, contain significantly a lower amount (10–30 vol%, on average) of smaller (0.3–3.0 mm) clasts of quartz, granite and other rocks. Thin (50–100 µm thick) lenticular montmorillonite aggregates, locally crossed by filaments of Mn and Fe oxides, are characteristic of the bedded carbonate–oxide manganese ore. The montmorillonite aggregates locally contain zigzag traces of possible benthic organisms’ activity.

The size of the rhodochrosite–rancieite aggregates varies, even within one thin section, from 0.1 to 0.4 mm. They occur in a fine and microporous mixture of Fe and Mn (?) oxyhydroxides with high variable Mn contents ranging from 0.2 to 0.4 atomic % (“manganese goethite”). The same oxide mixture locally occurs between the rancieite microdruses. The amount of “manganese goethite” varies widely. The ores contain numerous pores of varying sizes and shapes. Thin (10–30 µm) rims of rhomboid goethite crystals, closely intergrown with microfibrous montmorillonite, are widespread on the pore walls. Fibrous aggregates of Mn and Fe oxides are observed within the pores (Figure 7i) and always have a zonal structure: the axes of the filaments are composed either of rancieite or “manganese goethite”, while the edges are always composed of goethite. Most likely, the Mn and Fe oxide filaments formed in previously precipitated fine montmorillonite mass rather than in the pores. Montmorillonite was further washed out, as supported by the presence of montmorillonite clusters in some samples containing filamentous aggregates of Mn and Fe oxides. Sphalerite (wurtzite?), rutile (anatase?), ilmenite, zircon, titanite, epidote, chamosite, glauconite, albite, K-feldspar, apatite and monazite are accessory minerals in the carbonate–oxide ores.

In all textural varieties of the carbonate–oxide ores, Mn and Fe mineralization is fully hosted in the cement. Despite its heterogeneous structure, the paragenetic sequence of Mn and Fe mineral formation is as follows: (1) rhodochrosite (mechanical fragments of rhodochrosite spherulites) → (2) rancieite overgrowing the rhodochrosite clasts → (3) a mixture of Mn and Fe oxyhydroxides (“manganese goethite”) between the rhodochrosite–rancieite aggregates → (4) goethite encrustation of pores and filamentous aggregates of Mn–Fe ± Ca oxides in pores and montmorillonite clusters. The clasts of rhodochrosite spherulites from the carbonate–oxide ores are characterized by higher Mn contents (up to 86–98 mol% MnCO₃), similar to or even higher than those in the rhodochrosite spherulites in the carbonate ores.

4.2. Chemical Composition of Ores

The chemical composition of the iron and manganese ores is presented in Supplementary Tables S1–S3. The iron ores contain high amounts of SiO₂ and Fe₂O₃, as well as volatiles (H₂O and CO₂), as evidenced by the loss on ignition content (LOI). The total content of SiO₂ + Fe₂O₃+ FeO + LOI ranges from 85.06 to 99.04 wt%. The ores also contain Al₂O₃ (1.79–7.59 wt%), CaO (0.39–10.62 wt%), P₂O₅ (0.35–6.37 wt%) and MnO (0.57–2.43 wt%). The TiO₂, Na₂O, and K₂O contents are each <1 wt% (Table S1).

The composition of the carbonate manganese ores is characterized by the predominance of MnO and CO₂ (MnO + CO₂ = 72.38–78.69 wt%). The SiO₂, Fe₂O₃, Al₂O_3, Fe₂O₃, FeO, MgO and CaO contents are an order of magnitude lower, and the TiO₂, Na₂O, K₂O, and P₂O₅ contents are each <0.4 wt% (Table S1). The Mn/Fe ratio of the carbonate manganese ores (4.61–17.99) is ≥100 times higher than that of the iron ores and the average value for the crust [21]. Compared with the iron ores, the manganese ores contain more MgO.

The trace element patterns of all ore types from the Marsyaty deposit are similar, with only minor differences due to varying element concentrations (Figure 8, Table S2).

Among the trace elements analyzed, only the following elements have concentrations comparable to or higher than those in the upper crust: Co (0.69–2.61), Ni (0.71–3.05), Zn (1.13–4.01), Ge (1.38–9.07), As (2.39–21.88), Cd (2.00–14.40) and Sb (2.25–14.60) (expressed as C_i/C^{upper crust} values, where C_i and C^{upper crust} are the element concentrations in the studied ores and the upper crust [21], respectively). The contents of some elements can be both higher and lower than the reference values: Be (0.13–2.57), V (0.46–3.71), Cr (0.19–4.01), Cu (0.17–2.52), Sr (0.17–3.66), Y (0.29–1.56), Mo (0.32–3.82), W (0.22–2.09), Tl (0.02–5.75), Pb (0.33–2.83), Bi (0.24–7.22) and U (0.41–6.59). The concentrations of the other elements are most often below the upper crust levels: Li (0.24–1.34), Sc (0.13–0.84), Ga (0.22–0.81), Rb (0.04–0.12), Zr (0.14–0.50), Nb (0.03–0.15), Sn (0.08–0.24), Cs (0.05–0.24), Ba (0.13–1.00), Hf (0.07–0.24), Ta (0.06–0.11) and Th (0.11–0.51). The Se and Te contents are usually below the detection limit.

The total rare earth element (REE) content of the iron and manganese ores from the Marsyaty deposit (∑REE = 30.58–135.82 ppm) is comparable to or lower than that in the upper crust (∑REE = 139.58 ppm) (Figure 9). The upper crust-normalized [21] REE patterns are nearly flat and horizontal, with a slight slope towards the light REEs in the iron ores and a slight rise in the middle REEs in the manganese ores. The iron ores are slightly depleted in light REEs: (La/Lu)n = 0.30–0.44 and (La/Sm)n = 0.40–0.57. In manganese ores, this depletion is less striking: (La/Lu)n = 0.85–1.09 and (La/Sm)n = 0.70–0.76. The Eu and Ce anomalies are almost absent in all ore types: Eu/Eu* = 0.87–1.09, averaging 0.97, and Ce/Ce* = 0.86–1.20, averaging 1.05. Some iron ore samples exhibit a slight enrichment in Ce: Ce/Ce* = 1.10–1.20. The REE patterns of the iron oxide and iron carbonate ores are nearly identical.

Carbon isotopic composition of carbonates. All analyzed carbonates are enriched in the light carbon isotope ¹²C (

Table 2. Carbon isotopic composition of carbonates from the Marsyaty deposit.

No	Sample No	Ores/Lithological Variety	δ13Ccarb ‰, VPDB
	Iron oxide ores
1	90198-0	Loose quartz–smectite–goethite ochre	−21.2
2	90198-0b	Dense quartz sandstone with apatite–goethite cement	−21.2
3	90198-1c	Dense siderite oolitic ironstone	−22.7
4	90198-2b	Dense goethite oolitic ironstone	−24.8
	Iron carbonate ores
5	90198-0e	Dense siderite oolitic ironstone	−23.3
6	90198-2c		−18.5
	Manganese carbonate ores
7	Mr-1	Rhodochrosite ore with stromatolitic texture	−10.0
8	Mr-2		−12.2
	Manganese carbonate–oxide ores
9	90198-0f-1	Coarse-clastic rock with rancieite–goethite cement	−39.4
10	90198-0f-2		−41.0

Sampling locations are shown in Figure 3.

). In the iron ores, where siderite is a major carbonate, the δ¹³C_carb values (VPDB) vary within a relatively narrow range, from −24.8 to −18.5, with an average of −22.0‰.

Rhodochrosite from the manganese ores is characterized by a wider range of δ¹³C values. The highest values are typical of the carbonate ores (−12.2 to −10.0 ‰), which are almost twice as high as those of the carbonates from the iron ores. The minimum δ¹³C values (−41.0 to −39.4 ‰) in the studied deposit were determined for rhodochrosite spherulites in the manganese carbonate–oxide ores.

5. Discussion

The results of the geological, mineralogical and geochemical studies of the iron and manganese ores of the Marsyaty deposit indicate the following main issues of their genesis.

• The ores are sedimentary, as evidenced by their textural features: alternating coarse- and fine-clastic beds with varying contents of unaltered detritus and abundant organic remains with preserved cellular texture, as well as colloform, water-bearing Fe oxyhydroxides and phyllosilicates.
• The iron and manganese ores have different ages because they are separated by a bed of unsorted coarse-clastic sediments with detrital fragments of Mn carbonates.
• The earliest minerals of the iron ores include Fe oxyhydroxides (goethite) and chamosite, which compose the ooids. The presence of ooid clasts as the cores of new ooids suggests their formation under active hydrodynamic conditions. The authigenic minerals of the ooids and cement have similar compositions but different structures. These differences are likely due to different dynamic conditions during primary accumulation: more mobile ooids and more stable cement. In the manganese ores, the rhodochrosite spherulites are the earliest minerals.
• Siderite in the iron carbonate ores formed after the Fe oxyhydroxides and chamosite in the ooids. Siderite microspherulites in the cement show no signatures of replacement of the primary oxide and silicate–iron substrate, i.e., they could have formed in porous non-lithified sediment. The manganese carbonate–oxide ores contain fragments of rhodochrosite spherulites, which are similar to those in the manganese carbonate ores. The rhodochrosite spherulites were apparently sourced from the eroded carbonate–manganese strata.

Facies conditions of iron and manganese accumulation.

During the Mesozoic–Cenozoic, the West Siberian Platform was a large marine basin with adjacent coastal plains [22]. The land–sea boundary controlled the location of iron and manganese deposits (Figure 1), which accumulated from the Turonian to the Oligocene in various facies conditions, including floodplains, river deltas, lakes, and marine lagoons [7,15,23].

The Marsyaty deposit area was located on the western margin of the West Siberian Sea, near a large landmass of the Ural folded structures, which formed during the Late Permian–Early Triassic. By the Late Cretaceous, the eastern slope of the Urals has become an uplifted peneplain that transited into a lowland plain periodically flooded by the West Siberian Sea [14,22,23,24,25,26,27]. At least six main transgressive cycles, with smaller phases of sea-level fluctuations [11,25], have been recognized in the area of the manganese deposits in the Northern Urals across the Cretaceous–Paleogene boundary. The formation of the Marsyaty deposit is associated with one of these cycles.

The iron and manganese ores of the Marsyaty deposit occur at two different stratigraphic levels: the iron ores are hosted by Cenomanian rocks, whereas the manganese ores are hosted by Lower Paleocene rocks (Figure 2b,c). The facies sedimentation conditions of iron ores are ambiguous. The presence of wood fragments, broadleaf and conifer leaves, and the results of spore–pollen analyses of the iron ores indicate either a shallow marine or a continental lacustrine environment [11,26]. The well-rounded coarse-grained material of the iron ores, poor clast sorting, and oolitic structures are evidence of highly active hydrodynamic conditions during the formation of the iron-enriched sediments. The large thickness and length of the iron ore bed indicate high wave energy over a long period of time and over a large area. These conditions could have occurred in the coastal wave zone of a large water basin (a large lake or a bay) and possibly at the mouth of a large river. In any case, the accumulation of large masses of iron oxides under active hydrodynamic conditions required an effective geomorphic trap.

The iron ores are characterized by the presence of apatite–goethite cement in some lithological ore types. Phosphorus precipitates intensely in saline conditions than in freshwater basins [28]. Globular (peloidal) glauconite, which is widespread in the studied iron ores, also usually forms in marine settings [29,30,31,32] (and references therein). The assemblage of glauconite, Ca phosphates and Fe oxides is typical of marine sediments [33,34] (and references therein). Thus, the Fe-bearing sediments most likely accumulated in a large inner marine basin that was partly divided from the open sea by shallow banks, bars, etc.

Manganiferous rocks of the Northern Urals are considered to be marine shelf deposits [11,12,35]. Carbonate manganese ore deposits form at some distance from the land (the source of coarse-grained material) at shallow depths in a sedimentation zone of terrigenous sandy–clayey material above the storm or tidal wave base. Overall, the change from oolitic ironstones to carbonate manganese ores in the Marsyaty deposit indicates marine transgression.

The carbonate–oxide manganese ores of the Marsyaty deposit formed due to the erosion and redeposition of carbonate manganese ores. The primary carbonate ores were likely eroded during severe storms. The accumulation and burial of rhodochrosite detritus and newly formed Mn oxides in the cement occurred under conditions similar to those of oolitic ironstone formation in the coastal zone of a sea bay. The high content of medium- and coarse-grained lithogenic material in the carbonate–oxide manganese ores is similar to that of the oolitic ironstones, confirming this model. Thus, the rhodochrosite-bearing beds of carbonate–oxide manganese ores indicate shallow sedimentation conditions and are the products of disintegration of a carbonate deposit located far from the shore. Such areas could have formed during either the initial or the final phases of the transgressive cycle. For the Marsyaty deposit, where the carbonate–oxide manganese ores lie above the oolitic ironstones, the first scenario is more likely.

Geochemical features of ores.

Main elements.

The negative Si–Fe (r = –0.94; Figure 10a) and Si–Al (r = –0.90; Figure 10b) correlations in the ores reflect the changing ratio between detrital quartz and both authigenic Fe and clay minerals. In four iron ore samples, the Si/Al value of 2.00–3.10 is comparable to that of clays, while the higher Si/Al values of other samples (5.37–18.97) confirm a greater amount of quartz detritus. The positive correlation between Fe and Al (Figure 10c) indicates that the conditions for iron accumulation were also favorable for the accumulation of clayey material. It is possible that, during the formation of the primary Fe concentrations, fine-grained Fe oxyhydroxides co-precipitated with dust-like particles of clay minerals. The extremely high Al concentrations in the oolitic ironstones are partly explained by the presence of bean bauxite grains and kaolinite in the cement of the iron carbonate ore. The giant Devonian North Urals bauxite basin and the Mesozoic laterite weathering crust developed on the rocks of the Urals Orogen can be the source of the Al-enriched detrital material [22,36]. This is a specific feature of the Marsyaty deposit, in contrast to the smaller amount of Al-dominated minerals in, e.g., ironstones of the Bakchar [37] and Kamysh–Burun [10] deposits, where these minerals are absent.

The Al/Ti ratio is an important lithochemical indicator [21]. Both elements are primarily precipitated as detrital material, but Al is present in clays, whereas Ti is hosted by rutile and ilmenite in the silty–sandy fraction. For the upper crust, the average Al/Ti ratio is 42.17 [21]. Except for two Al/Ti ratios close to the crustal value (41.33 and 52.64), the Al/Ti ratios of most samples range from 5.85 to 35.80 (average: 21.00), which is almost two times lower than the average for the crust. Excluding one anomalously high TiO₂ concentration (0.48 wt%), Ti and Al exhibit a statistically significant positive correlation (Figure 10d), which is related to the changing amount of detrital material in the iron ores. The low Al/Ti values indicate that the accumulation of silty–sandy material was more intense compared with clayey material. This is typical of sediments formed in coastal areas of marine basins, where active wave action promotes the washing out of fine clay particles and the accumulation of heavy minerals such as leucoxene [38].

The P content of the studied iron ores exceeds the average crustal value (0.086 wt% P [21]) by 1.7–30.9 (average 9.8) times. High P contents are typical of oolitic iron ores from many deposits [7,8,9,10,39,40,41,42,43,44]. Apatite is the main P host in the oolitic ironstones [18]; thus, P and Ca have a positive correlation (Figure 10e). However, the average Ca/P ratio in the Marsyaty ores differ significantly from that in stoichiometric apatite, 5.0:2.3 and 5:3, respectively. This is apparently related to deviations in the Marsyaty apatite stoichiometry from the Ca₅(PO₄)₃(F,OH,Cl) standard. In particular, it can be associated with the replacement of Ca by Na, as indicated by the positive Ca–Na correlation (Figure 10f). Note also that the calculated Ca–P regression line intersects the P concentration axis at a value greater than zero (at Ca = 0, P > 0). This suggests that P in iron ores is also hosted by other minerals in addition to apatite, in particular, Fe oxyhydroxides, as well as monazite and perhamite [18,19].

The Mn concentrations in the iron ores exceed the average crustal values (0.077 wt % Mn [21]) by 4.6–23.4 (average 7.5 ) times. The Mn/Fe ratio of the iron ores ranges from 0.01 to 0.05 (average 0.02), which coincides with the average crustal Mn/Fe ratio of 0.02 [21]. Thus, iron accumulation was accompanied by an equivalent accumulation of manganese without significant differentiation. Elevated Mn concentrations of manganese are also determined in some other marine oolitic iron ore deposits [7,8,10]. The Mn/Fe ratio of the carbonate manganese ores (4.61–17.99) is ≥100 times higher than that of the iron ores and the average values for the crust.

Trace elements.

The Fe and Mn sedimentation conditions were also favorable for the accumulation of Co, Ni, Zn, Ge, As, Cd and Sb, the contents of which are higher than those in the upper crust (Ci/C^{upper crust} ≥ 1). These elements could have been introduced in dissolved or suspended form by continental (river, ground, etc.) waters or hydrothermal fluids (of any origin, including diagenetic), or they could have been absorbed from seawater and supplied with biogenic matter. It is unclear which scenario is more likely. However, statistically significant positive correlations between Fe, Ge, As and Sb in the iron ores (r_Fe–Me = 0.96, 0.85, and 0.83, respectively) are most likely explained by the intense sorption of these trace elements onto finely dispersed iron oxides.

Detrital aluminosilicate material was most likely the main source of trace elements with C_i/C^{upper crust} ratios < 1. Some trace elements are positively correlated with Al: Li (r = 0.75), Sc (r = 0.74), Ga (r = 0.92), Zr (r = 0.65), Nb (r = 0.87), Hf (r = 0.74) and Th (r = 0.89). The Al-normalized concentrations of Ga, Rb, Nb, Sn, Cs, Ba, Hf, Ta, Tl and Th ([C_i/C_Al] = [C_i/C_Al]^Sample/[C_i/C_Al^{upper crust}]) are less than or equal to 1 (Figure 8). The aluminosilicate material in the primary sediments was a source for these trace elements in the studied ores. Rubidium and Cs are positively correlated with K: (r = 0.84 and 0.85, respectively), which is explained by their substitution of K in feldspars and phyllosilicates.

The C_i/C^{upper crust} values of Be, V, Cr, Cu, Sr, Y, Mo, Pb, Bi and U vary from <1 to ≥1. For most of these elements, the C_i/C_Al values are >> 1, Consequently, the contents of these elements in the detrital component of the metalliferous sediments are insufficient. They were therefore additionally sourced from hydrogenic, biogenic, and possibly hydrothermal material or were intensely redistributed during diagenesis. Compared to Co, Ni, Zn, Ge, As, Cd and Sb, the accumulation of Be, V, Cr, Cu, Sr, Y, Mo, Pb, Bi and U were less intense. Therefore, their “excess” concentrations are “masked” by high contents of Fe and Mn, as well as volatiles, and are determined only after recalculation of the analyses (normalization to Al).

REE distribution.

The REE patterns of both the iron and manganese ores are generally typical of terrigenous and carbonate sediments and sedimentary rocks, as well as sedimentary–diagenetic iron and manganese ores [10,23,45,46,47,48,49,50,51]. The similar REE contents and shapes of the patterns of iron oxide and iron carbonate ores reflect their similar formation conditions. The REE concentrations are positively correlated with the Al content in the ores (r = 0.76); therefore, lithogenic aluminosilicate material was one of the REE sources in the sediments. The ∑REE/Al ratio of 24.39–83.30 in the ores is 1.4–4.7 times higher than that of the upper crust (∑REE/Al = 17.83), requiring an additional REE source for the ores in addition to the lithogenic material. The REEs could additionally be absorbed by Fe oxyhydroxides or clay minerals from seawater [52,53].

Carbon isotopic composition.

The δ¹³C_carb values of carbonates from the oolitic iron ore (−18.5 to −24.8 ‰) correspond to those of authigenic carbonates (δ¹³C_carb from −25 to −5 ‰) forming from carbon dioxide produced during microbial oxidation of organic matter (OM) in sediments during diagenesis or anadiagenesis [54,55,56,57]. Our values are close to the lower boundary of this interval because the oolitic ironstones accumulated in the coastal zone of the basin with a terrigenous sedimentation type. There were no conditions for carbonate precipitation, which would concentrate ¹³C dissolved in seawater. All reactive carbon precipitated as “sapropelic” (OM), which is characterized by low δ¹³C values. A similar C isotopic composition is established from oolitic ironstones from other deposits (−28.1 to −12.2‰ [56]).

The C isotopic composition of rhodochrosite from the manganese ores varies more widely (−41 to −10 ‰), most likely indicating the involvement of seawater ¹³C. Mixing of carbon from different sources resulted in higher δ¹³C_carb values. However, the amount of seawater ¹³C was relatively low; therefore, the C isotopic composition generally corresponds to that of biogenic carbonates. The lowest δ¹³C_carb values_. (−41.0 to −39.4 ‰) of the spherulitic rhodochrosite clasts from the carbonate–oxide manganese ore indicate CO₂ production during microbial oxidation of biogenic methane in sediments during early diagenesis [49]. Authigenic carbonates, including manganese ones, formed by methane oxidation are found in sediments of both present-day and ancient seas [58,59,60,61,62,63,64]. Such carbonates are most commonly indicators of seafloor methane seeps zones. This is most likely the first report of methane isotopic signatures in manganese carbonates from the Northern Urals.

Iron and manganese sources.

The area of the Marsyaty deposit is hosted exclusively by sedimentary rocks without any igneous (including volcanic) complexes. Thus, the ore elements could have been sourced from: (1) rivers and groundwater from continental weathering crusts, (2) seafloor seeps of exfiltration (elision) fluids and (3) bottom currents directed from the deep parts of the marine basin toward the shallow area [2,6,7,9,37,65,66,67,68,69,70,71,72,73,74].

All these scenarios could explain the formation of the Marsyaty deposit. For the oolitic ironstones, which contain abundant detrital material, we can suggest a dominant continental source of Fe, Mn and some other elements. Iron and Mn could have been supplied together, as the average Mn/Fe ratio of the iron ores of 0.02 coincides with the average crustal value [21].

The Mn source of the carbonate manganese ores is not so obvious. These ores formed far from the coastline after the onset of marine transgression. The amount of fine- and medium-clastic material is low, and their ore textures indicate biogenic manganese accumulation under low background sedimentation rates. The Mn/Fe ratio of the manganese ores is two to three orders of magnitude higher than that of iron ores and the average crustal ratio. The “excessive” Mn in manganese ores possibly have different sources. It is noteworthy, however, that the iron and manganese ores exhibit similar trace element patterns, with a few exceptions, e.g., Tl is hosted by the manganese ores, while the iron ores are Tl-poor (Figure 8). Similar trace element patterns of iron and manganese ores most likely indicate a common source for most trace elements (terrigenous material). The REE patterns of iron and manganese ores are different (Figure 9), which can indicate either an additional REE source or another REE behavior during absorption by Mn oxides.

Formation model of the Marsyaty deposit.

The geochemical conditions of Fe and Mn accumulation in sedimentary strata are well studied [2,3,5,6,9,75]. Changes in Eh and/or pH of the solution control the precipitation and dissolution of Fe and Mn. Both metals are soluble in reducing and acidic environments, but precipitate with increasing oxidation potential and alkalinity. Compared with Fe, Mn precipitates in more oxidizing and/or more alkaline environments. These different precipitation conditions facilitate metal differentiation, which can occur at any stage of sedimentation: during (i) the selective leaching of Fe and/or Mn from source rocks, (ii) metal transport in solution or (iii) accumulation in sediments. The most complete differentiation of Fe and Mn occurs in calm hydrodynamic conditions, with a slow flow of ore material to the water basin and a gradual change in Eh and pH of the waters. Depending on the geological setting, these conditions lead to the formation of either zoned Fe–Mn deposits or to the dispersion of one of the metals (Fe or Mn) into the ambient seawater, while the other one forms a deposit. However, if one of the above conditions is absent, no iron and manganese differentiation occurs or is minor, and metalliferous sediments are characterized by low Mn/Fe values. In the primary sediments, Fe and Mn mainly accumulate in the form of Fe³⁺, Mn³⁺ and Mn⁴⁺ oxides/oxyhydroxides, whereas further anadiagenesis results in the formation of Mn silicates and Fe and Mn carbonates.

Based on the results of the geological, mineralogical and geochemical studies, the formation of metalliferous sediments of the Marsyaty deposit can be presented as follows (Figure 11). Regardless of the metal sources, the ferruginous sediments (Mn/Fe ≈ 0.02) formed in the coastal zone of the marine basin. The intensive ore matter influx and abrupt changes in physicochemical conditions, together with active seawater hydrodynamics, favored the joint Fe and Mn accumulation in the sediments. Constant wave movements and periodic storms contributed to the formation of oolitic structure of the iron oxide sediments, regular rewashing and redeposition of the metalliferous sediments, and their dilution with coarse- and fine-grained (including clays) material. As a result of post-sedimentation processes, partial replacement of OM with carbonates, silica and phosphates occurred.

During diagenesis, the decomposition of reactive OM buried in the sediments led to the absorption of oxygen from the pore solution and the formation of CO₂. A deficit of OM or partial mixing of sediments with bottom waters led to moderately oxidizing conditions with low CO₂ concentrations in sediments, facilitating the reaction of Fe oxyhydroxide with quartz and kaolinite to form chamosite. This reaction, coupled with the low Al content in the sediment, resulted in the formation of the quartz + chamosite + goethite assemblage typical of the iron oxide ores, which occurs both in and between the oolites. Diagenesis also leads to the local redistribution of Mn, which was mostly incorporated into the composition of interstitial goethite and formed less abundant rhodochrosite grains.

The initially higher OM content of the sediments resulted in reducing conditions, with much lower oxygen concentrations in the pore solution, whereas the CO₂ content increased sufficiently to produce stable Fe carbonates instead of Fe oxides. This led first to the replacement of goethite and chamosite–goethite ooides by siderite and the crystallization of interstitial siderite between the ooides, resulting in the formation of mostly carbonate (± quartz and kaolinite) iron ores. The diagenetic origin of siderite, which formed after Fe oxyhydroxides and silicates, is evident from the replacement of concentrically zoned ooids by radial crystalline siderite. The participation of biogenic CO₂ in its formation is confirmed by its C isotopic composition (see above). Physicochemical calculations also show that the formation of siderite requires relatively high CO₂ concentrations, which are available in relatively closed areas of the sediment that prevent the penetration of new portions of oxygen-rich seawaters [75]. The replacement of goethite by siderite releases Si, Al and P, which were initially absorbed by goethite, and facilitates the formation of perhamite and some other minerals [19].

The manganese-enriched sediments (Mn/Fe >> 1) accumulated in calm hydrodynamic conditions with a relatively low rate of accumulation of detrital material and almost without Fe (Figure 11b), reflecting different physicochemical conditions.

In modern water basins, Mn typically accumulates primarily as Mn³⁺/Mn⁴⁺ oxides rather than as Mn carbonates due to insufficient CO₂ concentration in seawater, even in stagnant areas [76]. Mn carbonates (rhodochrosite and kutnahorite) formed during diagenesis through the reaction of primary sedimentary Mn oxides with CO₂ produced during microbial OM decomposition [5,76]. The composition and structure of the carbonate manganese ores of the Marsyaty deposit are generally consistent with this idea. These ores are characterized by typical diagenetic spherulitic rhodochrosite aggregates, and their C isotope composition indicates the involvement of microbiota in their formation.

At the same time, the carbonate manganese ores of the Marsyaty deposit lack any relics of primary sedimentary Mn oxides, most likely because they formed fine particles, which were completely replaced by rhodochrosite during early diagenesis. The carbonate manganese ores exhibit wavy-banded stromatolite-like structures, and the rhodochrosite contains intergrowths of concentrically zoned spherulites interpreted as microstromatolites. Together with the isotopic data, the stromatolite-like structure reflects the direct involvement of a biological community in the formation of the Mn carbonates. These carbonate manganese ores likely hosted organic (bacterial–algal?) structures that formed within unconsolidated sediment near the seafloor. A benthic biocenosis developed through phyto- and chemosynthesis processes associated with the precipitation of Mn oxides and their subsequent transformation into rhodochrosite. The presence of such biocenoses is substantiated by both geological and geochemical data, as well as by the results of laboratory experiments [77,78,79,80,81,82]. Isotopic data indicate that at least part of the organic manganese structures of the Marsyaty deposit could have arisen in methane seepage areas.

The carbonate manganese edifices formed at the water–sediment interface. They were porous, unlithified sediments and were easily eroded during strong storms; thus, rhodochrosite clasts accumulated on the flanks of the carbonate deposits. Partial oxidation of rhodochrosite in oxygen-rich bottom waters led to the fast formation and further burial of manganese carbonate–oxide ores, as evidenced by the numerous relics of rhodochrosite spherulites in the rocks. Full oxidation of rhodochrosite was apparently prevented by the formation of a dense crust of newly formed rancieite. It is possible that erosion of the underlying oolitic iron ores also contributed to the formation of manganese carbonate–oxide ores, which is indicated by the abundant “manganese goethite” in the cement of these ores.

6. Conclusions

Oolitic ironstones and manganese ores of the Marsyaty deposit (Northern Urals, Russia) are spatially combined and divided by a sand and gravelite interbed. The oolitic ironstones consist mostly of alternating primary Fe oxyhydroxides and chamosite, with overprinted siderite. The oolitic ironstones formed in oxidizing coastal conditions. The δ¹³C_carb values of siderite (−18.5 to −24.8‰, VPDB) indicate a biogenic CO₂ source associated with the oxidation of buried organic matter. Primary manganese carbonate ores formed in the presence of biota, as evidenced by the stromatolite-like structures of the manganese ores. Extremely low δ¹³C_carb values (up to −41.0‰, VPDB) of rhodochrosite may indicate methane as the carbon source.

Sedimentary manganese and iron deposits are typically separated in space because of the different facies and physicochemical conditions required for iron and manganese sedimentation. This leads to different distributions of sedimentary oolitic ironstones and manganese ores. Oolitic ironstone deposits free of manganese ores are widespread on the periphery of the Cretaceous–Paleogene West Siberian sedimentary basin, while manganese deposits were found only along the eastern slope of the Northern Urals. Iron and manganese ores coexist at the studied Marsyaty deposit. The iron and manganese ores of the Marsyaty deposit belong to a common transgressive series of sedimentary rocks overlying each other. The iron and manganese ores formed sequentially during the evolution of a marine basin. The degree of elemental differentiation, indicated by the Mn/Fe ratio, increases with rising sea level: iron precipitated mostly in the coastal zone, while manganese precipitated in the shallow shelf. A unique feature of the Marsyaty deposit includes two ore formation stages within a limited area and over a relatively short geological period: the accumulation of (i) oolitic ironstones enriched in Mn first and then (ii) manganese ores only.